Combine brown sugar, corn syrup, and butter in a medium saucepan.

Cook over medium heat, stirring constantly until sugar is completely dissolved and the mixture comes to a full rolling boil.

Allow the mixture to boil, without stirring, for exactly 1 minute. 

Remove from heat and let stand for 5 minutes.

Combine evaporated milk, baking soda, and vanilla in a bowl.

Stir into slightly cooled sauce.

Pour into a jar, cover, and refrigerate.